Fuel oil and natural gas shortages have sparked renewed world-wide interest in the development of processes that can produce clean synthetic natural gas, or gas or pipeline quality, from carbonaceous solids, particularly coal. Various processes, both thermal and catalytic, are known for the gasification of coal to produce pipeline quality gas. In gasification processes of these types, raw gas mixtures are produced which include hydrogen, carbon monoxide and methane. Since methane, the desired high BTU component, cannot normally be directly produced within the gaseous product in adequate concentrations to provide a pipeline quality gas, the raw gases are upgraded in separate downstream reactors, and processing units to produce additional methane.
A raw gas, after separation of the methane component, is conventionally upgraded downstream of the main reactor in a series of shift-methanation reactions which increases the methane content of the gas. In a shift reaction, additional hydrogen is first generated by reacting carbon monoxide and steam to produce carbon dioxide and hydrogen, as characterized by the equation: CO + H.sub.2 O.fwdarw. CO.sub.2 + H.sub.2. After removal of acid components (e.g., CO.sub.2), the hydrogen is then reacted with carbon monoxide in a methanation reaction to produce methane, as characterized by the equation: CO + 3H.sub.2 .fwdarw.CH.sub.4 + H.sub.2 O. There are, of course, many variables which determine the efficiency of any given gasification process. The amount of methane which can be directly produced in any given gasification process vis-a-vis that which can be indirectly produced, however, is an important variable in determining the efficiency of a coal gasification process.
Although both thermal and catalytic coal gasification processes have been generally known for many years, at least until recently, neither type of process had proven of outstanding efficiency, one type relative to the other. Each had its advantages and its disadvantages. Various designs of each process type are thus being widely investigated, and developed for possible commercial use. A catalytic process of admirable merit is described in Application Ser. No. 514,852, filed Oct. 15, 1974, by K. K. Koh et al. and not abandoned. In this process, herewith incorporated by reference, methane is produced in a catalytic gasification zone, suitably one containing a fluidized bed, by reacting steam with carbonaceous solids, particularly coal, in the presence of a carbon-alkali metal catalyst, or an alkali-metal impregnated carbonaceous feed, and a recycle stream of synthesis gas (H.sub.2 + CO). The catalytic gasification reaction is conducted at temperatures ranging about 1000.degree. to 1600.degree. F, and at pressures ranging about 100 to 1500 pounds per square inch absolute (psia). A feature of this process is that completing reactions are suppressed by the recycle or synthesis gas, such that the net reaction products are essentially methane and carbon dioxide, in accordance with the equation: 2C + 2H.sub.2 O.fwdarw.CH.sub.4 + CO.sub.2. Product methane, carbon dioxide, and the synthesis gas used as recycle to suppress competing reactions are withdrawn from the gasifier, passed through a heat recovery system, and then sent to a cryogenic separation unit from which the methane and carbon dioxide are separately recovered. In a typical coal, 1 mole of methane that is recovered contains 98.7% of the energy of the 2 moles of carbon gasified.
The Koh et al. catalytic process offers profound advantages over prior art processes, both thermal and catalytic. Whereas the thermal efficiency of thermal gasification processes, in particular, is severely limited by the sequence of reactions, such limitations can be avoided by the Koh et al. catalytic process. A gas of high methane content can be directly produced. The gasification can be conducted at high rate, even at relatively low temperature. The reaction is substantial thermoneutral and, although it is necessary to heat the reactants to reaction temperature to initiate the reaction, essentially all of the heat supplied to the reactor is recovered. Hence, there is very little waste heat. The discovery that methane and synthesis gas can be equilibrated in the reaction, i.e., maintained in equilibrium by separation and recycle of synthesis gas to the reactor to produce methane directly, has eliminated all need for downstream shift-methanation reactions, and the thermal efficiency of this process is significantly higher than that of prior art processes.
Despite the relatively high efficiency, and other advantages offered by this process, a further deficiency resides in the less than total gasification of the feed carbon. A major source of carbon loss is caused by backmixing of particles, and by the elutriation of fines present in the feed coal, inclusive of the elutriation of fines created by attrition within the gasifier. Not only does the loss of the elutriated fines lower the amount of carbon conversion that is possible, but this also results in high catalyst losses, which effect is particularly manifest because a considerably greater weight proportion of catalyst is contained in the entrained fines than in the initial feed.